Fixed deterministic post-backoff for cyclic prioritized multiple access (CPMA) contention-free sessions

ABSTRACT

A cyclic prioritized multiple access (CPMA) method is disclosed which includes Fixed Deterministic Post-Backoff. Fixed deterministic post-backoff reduces conflicts between access points of overlapping cells. Contention-free sessions (CFSs) can be generated, one from each overlapping cell. Each active access point engages in a fixed deterministic post-backoff. A fixed deterministic backoff delay (Bkoff times a fixed number of idle time slots) is used by all access points, with the value of Bkoff being greater than the number of overlapping cells. The Bkoff should be large enough to enable the traffic that needs to be accommodated by the channel. Each access point has a backoff timer that is counted down using the shortest interframe space possible, typically the Priority Interframe Space (PIFS). A contention-free session (CFS) is initiated when the backoff timer expires, and it is then reset to the value of Bkoff to start a new cycle. A cycle is measured in terms of idle time slots instead of a fixed time interval. Contention-based transmissions can be attempted by an access point or other stations in the cell using their assigned priority while the access point is counting down its backoff timer. A new access point can get started and resolve possible collisions by a small random backoff. Subsequent contention-free sessions (CFSs) will not conflict, given an existing sequence of non-conflicting CFSs, since the follower access point&#39;s backoff delay exceeds that of the leader&#39;s by at least one times the fixed number of idle time slots. In this manner, contention-free sessions can be conducted without interference in the first and second cells.

[0001] This application claims the benefit of the following co-pendingapplications:

[0002] [1] U.S. Provisional Application Serial No. 60/330,930, filedNov. 2, 2001, entitled “HCF ACCESS MECHANISM: OBSS MITIGATION,”

[0003] [2] U.S. Provisional Application Serial No. 60/331,030, Nov. 7,2001, entitled “‘NEIGHBORHOOD’ CAPTURE IN CSMA/CA WLANS,”

[0004] [3] U.S. Provisional Application Serial No. 60/331,211 Nov. 13,2001, entitled “‘SHIELD’: PROTECTING HIGH PRIORITY CHANNEL ACCESSATTEMPTS,” and

[0005] [4] U.S. Provisional Application Serial No. 60/342,343, Dec. 21,2001, entitled “WIRELESS LANS AND ‘NEIGHBORHOOD CAPTURE’,” all of whichare incorporated herein by reference.

RELATED APPLICATIONS

[0006] This patent application is related to the copending regular U.S.patent application Ser. No. 09/985,257, filed Nov. 2, 2001, by MathildeBenveniste, entitled “TIERED CONTENTION MULTIPLE ACCESS (TCMA): A METHODFOR PRIORITY-BASED SHARED CHANNEL ACCESS,” which is incorporated byreference.

[0007] This patent application is also related to the copending regularU.S. patent application Ser. No. 10/187,132, filed Jun. 28, 2002, byMathilde Benveniste, entitled “HYBRID COORDINATION FUNCTION (HCF) ACCESSTHROUGH TIERED CONTENTION AND OVERLAPPED WIRELESS CELL MITIGATION,”which is incorporated by reference.

[0008] This patent application is also related to the copending regularU.S. patent application Ser. No. ______, filed ______, 2002 by MathildeBenveniste, entitled “‘SHIELD’: PROTECTING HIGH PRIORITY CHANNEL ACCESSATTEMPTS IN OVERLAPPED WIRELESS CELLS,” which is incorporated byreference.

[0009] This patent application is also related to the copending regularU.S. patent application Ser. No. ______, filed ______, 2002, by MathildeBenveniste, entitled “WIRELESS LANS AND NEIGHBORHOOD CAPTURE,” which isincorporated by reference.

[0010] This patent application is also related to the copending regularU.S. patent application Ser. No. ______, filed ______, 2002, by MathildeBenveniste, entitled “STAGGERED STARTUP FOR CYCLIC PRIORITIZED MULTIPLEACCESS (CPMA) CONTENTION-FREE SESSIONS,” which is incorporated byreference.

[0011] This patent application is also related to the copending regularU.S. patent application Ser. No. ______, filed ______, 2002, by MathildeBenveniste, entitled “ACCESS METHOD FOR PERIODIC CONTENTION-FREESESSIONS,” which is incorporated by reference.

[0012] This patent application is also related to the copending regularU.S. patent application Ser. No. ______, filed ______, 2002, by MathildeBenveniste, entitled “PREEMPTIVE PACKET FOR MAINTAINING CONTIGUITY INCYCLIC PRIORITIZED MULTIPLE ACCESS (CPMA) CONTENTION-FREE SESSIONS,”which is incorporated by reference.

FIELD OF THE INVENTION

[0013] The invention disclosed broadly relates to telecommunicationsmethods and more particularly relates to wireless cells that haveoverlapping stations contending for the same medium.

BACKGROUND OF THE INVENTION Wireless Local Area Networks (WLANs)

[0014] Wireless local area networks (WLANs) generally operate at peakspeeds of between 10 to 100 Mbps and have a typical range of 100 meters.Single-cell wireless LANs are suitable for small single-floor offices orstores. A station in a wireless LAN can be a personal computer, a barcode scanner, or other mobile or stationary device that uses a wirelessnetwork interface card (NIC) to make the connection over the RF link toother stations in the network. The single-cell wireless LAN providesconnectivity within radio range between wireless stations. An accesspoint allows connections via the backbone network to wired network-basedresources, such as servers. A single-cell wireless LAN can typicallysupport up to 25 users and still keep network access delays at anacceptable level. Multiple-cell wireless LANs provide greater range thandoes a single cell through means of a set of access points and a wirednetwork backbone to interconnect a plurality of single-cell LANs.Multiple-cell wireless LANs can cover larger multiple-floor buildings. Amobile laptop computer or data collector with a wireless networkinterface card (NIC) can roam within the coverage area while maintaininga live connection to the backbone network.

[0015] Wireless LAN specifications and standards include the IEEE 802.11Wireless LAN Standard and the HIPERLAN Type 1 and Type 2 Standards. TheIEEE 802.11 Wireless LAN Standard is published in three parts as IEEE802.11-1999, IEEE 802.11a-1999, and IEEE 802.11b-1999, which areavailable from the IEEE, Inc. web sitehttp://grouper.ieee.org/groups/802/11. An overview of the HIPERLAN Type1 principles of operation is provided in the publication HIPERLAN Type 1Standard, ETSI ETS 300 652, WA2 December 1997. An overview of theHIPERLAN Type 2 principles of operation is provided in the BroadbandRadio Access Network's (BRAN) HIPERLAN Type 2; System Overview, ETSI TR101 683 VI.I.1 (2000-02) and a more detailed specification of itsnetwork architecture is described in HIPERLAN Type 2, Data Link Control(DLC) Layer; Part 4. Extension for Home Environment, ETSI TS 101 761-4V1.2.1 (2000-12). A subset of wireless LANs is Wireless Personal AreaNetworks (PANs), of which the Bluetooth Standard is the best known. TheBluetooth Special Interest Group, Specification Of The Bluetooth System,Version 1.1, Feb. 22, 2001, describes the principles of Bluetooth deviceoperation and communication protocols.

[0016] The IEEE 802.11 Wireless LAN Standard defines at least twodifferent physical (PHY) specifications and one common medium accesscontrol (MAC) specification. The IEEE 802.11(a) Standard is designed tooperate in unlicensed portions of the radio spectrum, usually either inthe 2.4 GHz Industrial, Scientific, and Medical (ISM) band or the 5 GHzUnlicensed-National Information Infrastructure (U-NII) band. It usesorthogonal frequency division multiplexing (OFDM) to deliver up to 54Mbps data rates. The IEEE 802.11(b) Standard is designed for the 2.4 GHzISM band and uses direct sequence spread spectrum (DSSS) to deliver upto 11 Mbps data rates. The IEEE 802.11 Wireless LAN Standard describestwo major components, the mobile station and the fixed access point(AP). IEEE 802.11 networks can also have an independent configurationwhere the mobile stations communicate directly with one another, withoutsupport from a fixed access point.

[0017] A single-cell wireless LAN using the IEEE 802.11 Wireless LANStandard is an Independent Basic Service Set (IBSS) network. An IBSS hasan optional backbone network and consists of at least two wirelessstations. A multiple-cell wireless LAN using the IEEE 802.11 WirelessLAN Standard is an Extended Service Set (ESS) network. An ESS satisfiesthe needs of large coverage networks of arbitrary size and complexity.

[0018] Each wireless station and access point in an IEEE 802.11 wirelessLAN implements the MAC layer service, which provides the capability forwireless stations to exchange MAC frames. The MAC frame transmitsmanagement, control, or data between wireless stations and accesspoints. After a station forms the applicable MAC frame, the frame's bitsare passed to the Physical Layer for transmission.

[0019] Before transmitting a frame, the MAC layer must first gain accessto the network. Three interframe space (IFS) intervals defer an IEEE802.11 station's access to the medium and provide various levels ofpriority. Each interval defines the duration between the end of the lastsymbol of the previous frame to the beginning of the first symbol of thenext frame. The Short Interframe Space (SIFS) provides the highestpriority level by allowing some frames to access the medium beforeothers, such as an Acknowledgement (ACK) frame, a Clear-to-Send (CTS)frame, or a subsequent fragment burst of a previous data frame. Theseframes require expedited access to the network to minimize frameretransmissions.

[0020] The Priority Interframe Space (PIFS) is used for high-priorityaccess to the medium during the contention-free period. A pointcoordinator in the access point connected to the backbone networkcontrols the priority-based Point Coordination Function (PCF) to dictatewhich stations in the cell can gain access to the medium. The pointcoordinator in the access point sends a contention-free poll frame to astation, granting the station permission to transmit a single frame toany destination. All other stations in the cell can only transmit duringa contention-free period if the point coordinator grants them access tothe medium. The end of the contention-free period is signaled by thecontention-free end frame sent by the point coordinator, which occurswhen time expires or when the point coordinator has no further frames totransmit and no stations to poll. The Priority Interframe Space (PIFS)is also known as the PCF Interframe Space.

[0021] The distributed coordination function (DCF) Interframe Space(DIFS) is used for transmitting low priority data frames during thecontention-based period. The DIFS spacing delays the transmission oflower priority frames to occur later than the priority-basedtransmission frames. An Extended Interframe Space (EIFS) goes beyond thetime of a DIFS interval as a waiting period when a bad reception occurs.The EIFS interval provides enough time for the receiving station to sendan acknowledgment (ACK) frame.

[0022] During the contention-based period, the distributed coordinationfunction (DCF) uses the Carrier-Sense Multiple Access With CollisionAvoidance (CSMA/CA) contention-based protocol, which is similar to IEEE802.3 Ethernet. The CSMA/CA protocol minimizes the chance of collisionsbetween stations sharing the medium by waiting a random backoff intervalif the station's sensing mechanism indicates a busy medium. The periodof time a minimal interval following traffic on the medium is when thehighest probability of collisions occurs, especially where there is highutilization. Once the medium is idle, CSMA/CA protocol causes eachstation to delay its transmission by a random backoff time, therebyminimizing the chance it will collide with those from other stations.

[0023] The CSMA/CA protocol computes the random backoff time as theproduct of a constant, the slot time, times a pseudo-random number RNthat has a range of values from zero to a collision window CW. The valueof the collision window for the first try to access the network is CW1,which yields the first-try random backoff time. If the first try toaccess the network by a station fails, then the CSMA/CA protocolcomputes a new CW by doubling the current value of CW as CW2=CW1 times2. The value of the collision window for the second try to access thenetwork is CW2, which yields the second-try random backoff time. Thisprocess by the CSMA/CA protocol of increasing the delay beforetransmission is called binary exponential backoff. The reason forincreasing CW is to minimize collisions and maximize throughput for bothlow and high network utilization. Where there is a low networkutilization, stations are not forced to wait very long beforetransmitting their frame. On the first or second attempt, a station willmake a successful transmission. However, if the utilization of thenetwork is high, the CSMA/CA protocol delays stations for longer periodsto avoid the chance of multiple stations transmitting at the same time.If the second try to access the network fails, then the CSMA/CA protocolcomputes a new CW by again doubling the current value of CW as CW3=CW1times 4. The value of the collision window for the third try to accessthe network is CW3, which yields the third-try random backoff time. Thevalue of CW increases to relatively high values after successiveretransmissions under high traffic loads. This provides greatertransmission spacing between stations waiting to transmit.

Collision Avoidance Techniques

[0024] Four general collision avoidance approaches have emerged: [1]Carrier Sense Multiple Access (CSMA) [see, F. Tobagi and L. Kleinrock,“Packet Switching in Radio Channels: Part I—Carrier Sense MultipleAccess Models and their Throughput Delay Characteristics,” IEEETransactions on Communications, Vol. 23, No. 12, pp. 1400-1416, 1975],[2] Multiple Access Collision Avoidance (MACA) [see, P. Karn, “MACA—ANew Channel Access Protocol for Wireless Ad-Hoc Networks,” Proceedingsof the ARRL/CRRL Amateur Radio Ninth Computer Networking Conference, pp.134-140, 1990], [3] their combination CSMA/CA, and [4] collisionavoidance tree expansion.

[0025] CSMA allows access attempts after sensing the channel foractivity. Still, simultaneous transmit attempts lead to collisions, thusrendering the protocol unstable at high traffic loads. The protocol alsosuffers from the hidden terminal problem.

[0026] The latter problem was resolved by the Multiple Access CollisionAvoidance (MACA) protocol, which involves a three-way handshake. [P.Karn, supra.] The origin node sends a request-to-send (RTS) notice ofthe impending transmission. A response is returned by the destination ifthe RTS notice is received successfully and the origin node proceedswith the transmission. This protocol also reduces the average delay ascollisions are detected upon transmission of merely a short message, theRTS. With the length of the packet included in the RTS and echoed in theclear-to-send (CTS) messages, hidden terminals can avoid colliding withthe transmitted message. However, this prevents the back-to-backre-transmission in case of unsuccessfully transmitted packets. Afive-way handshake Multiple Access Collision Avoidance (MACA) protocolprovides notification to competing sources of the successful terminationof the transmission. [See, V. Bharghavan, A. Demers, S. Shenker, and L.Zhang, “MACAW: A media access protocol for wireless LANs,” SIGCOMM '94,pp. 212-225, ACM, 1994.]

[0027] CSMA and MACA are combined in CSMA/CA, which is MACA with carriersensing, to give better performance at high loads. A four-way handshakeis employed in the basic contention-based access protocol used in theDistributed Coordination Function (DCF) of the IEEE 802.11 Standard forWireless LANs. [See, IEEE Standards Department, D3, “Wireless MediumAccess Control and Physical Layer WG,” i IEEE Draft Standard P802.11Wireless LAN, January 1996.]

[0028] Collisions can be avoided by splitting the contending terminalsbefore transmission is attempted. In the pseudo-Bayesian control method,each terminal determines whether it has permission to transmit using arandom number generator and a permission probability “p” that depends onthe estimated backlog. [See, R. L. Rivest, “Network Control by BayesianBroadcast,” IEEE Trans. Inform. Theory, Vol. IT 25, pp. 505-515,September 1979.]

[0029] To resolve collisions, subsequent transmission attempts aretypically staggered randomly in time using the following two approaches:binary tree and binary exponential backoff.

[0030] Upon collision, the binary tree method requires the contendingnodes to self-partition into two groups with specified probabilities.This process is repeated with each new collision. The order in whichcontending nodes transmit is determined either by serial or parallelresolution of the tree. [See, J. L. Massey, “Collision-ResolutionAlgorithms and Random-Access Communications,” in Multi-UserCommunication Systems, G. Longo (ed.), CISM Courses and Lectures No.265, New York: Springer 1982, pp.73-137.]

[0031] In the binary exponential backoff approach, a backoff countertracks the number of pauses and hence the number of completedtransmissions before a node with pending packets attempts to seize thechannel. A contending node initializes its backoff counter by drawing arandom value, given the backoff window size. Each time the channel isfound idle, the backoff counter is decreased and transmission isattempted upon expiration of the backoff counter. The window size isdoubled every time a collision occurs, and the backoff countdown startsagain. [See, A. Tanenbaum, Computer Networks, 3^(rd) ed., Upper SaddleRiver, N.J., Prentice Hall, 1996.] The Distributed Coordination Function(DCF) of the IEEE 802.11 Standard for Wireless LANs employs a variant ofthis contention resolution scheme, a truncated binary exponentialbackoff, starting at a specified window and allowing up to a maximumbackoff range below which transmission is attempted. [IEEE StandardsDepartment, D3, supra.] Different backoff counters may be maintained bya contending node for traffic to specific destinations. [Bharghavan,supra.]

[0032] In the IEEE 802.11 Standard, the channel is shared by acentralized access protocol, the Point Coordination Function (PCF),which provides contention-free transfer based on a polling schemecontrolled by the access point (AP) of a basic service set (BSS). [IEEEStandards Department, D3, supra.] The centralized access protocol gainscontrol of the channel and maintains control for the entirecontention-free period by waiting a shorter time between transmissionsthan the stations using the Distributed Coordination Function (DCF)access procedure. Following the end of the contention-free period, theDCF access procedure begins, with each station contending for accessusing the CSMA/CA method.

[0033] The 802.11 MAC Layer provides both contention and contention-freeaccess to the shared wireless medium. The MAC Layer uses various MACframe types to implement its functions of MAC management, control, anddata transmission. Each station and access point on an 802.11 wirelessLAN implements the MAC Layer service, which enables stations to exchangepackets. The results of sensing the channel to determine whether themedium is busy or idle are sent to the MAC coordination function of thestation. The MAC coordination also carries out a virtual carrier-senseprotocol based on reservation information found in the Duration Field ofall frames. This information announces to all other stations the sendingstation's impending use of the medium. The MAC coordination monitors theDuration Field in all MAC frames and places this information in thestation's Network Allocation Vector (NAV) if the value is greater thanthe current NAV value. The NAV operates similarly to a timer, startingwith a value equal to the Duration Field of the last frame transmissionsensed on the medium and counting down to zero. After the NAV reacheszero, the station can transmit if its physical sensing of the channelindicates a clear channel.

[0034] At the beginning of a contention-free period, the access pointsenses the medium; and if it is idle, it sends a beacon packet to allstations. The beacon packet contains the length of the contention-freeinterval. The MAC coordination in each member station places the lengthof the contention-free interval in the station's Network AllocationVector (NAV), which prevents the station from taking control of themedium until the end of the contention-free period. During thecontention-free period, the access point can send a polling message to amember station, enabling it to send a data packet to any other stationin the BSS wireless cell.

Quality of Service (QoS)

[0035] Quality of service (QoS) is a measure of service quality providedto a customer. The primary measures of QoS are message loss, messagedelay, and network availability. Voice and video applications have themost rigorous delay and loss requirements. Interactive data applicationssuch as Web browsing have less restrained delay and loss requirements,but they are sensitive to errors. Non-real-time applications such asfile transfer, email, and data backup operate acceptably across a widerange of loss rates and delay. Some applications require a minimumamount of capacity to operate at all—for example, voice and video. Manynetwork providers guarantee specific QoS and capacity levels through theuse of Service-Level Agreements (SLAs). An SLA is a contract between anenterprise user and a network provider that specifies the capacity to beprovided between points in the network that must be delivered with aspecified QoS. If the network provider fails to meet the terms of theSLA, then the user may be entitled a refund. The SLA is typicallyoffered by network providers for private line, frame relay, ATM, orInternet networks employed by enterprises.

[0036] The transmission of time-sensitive and data application trafficover a packet network imposes requirements on the delay or delay jitter,and the error rates realized; these parameters are referred togenerically as the QoS (Quality of Service) parameters. Prioritizedpacket scheduling, preferential packet dropping, and bandwidthallocation are among the techniques available at the various nodes ofthe network, including access points, that enable packets from differentapplications to be treated differently, helping achieve the differentquality of service objectives. Such techniques exist in centralized anddistributed variations.

[0037] Management of contention for the shared transmission medium mustreflect the goals sought for the performance of the overall system. Forinstance, one such goal would be the maximization of goodput (the amountof good data transmitted as a fraction of the channel capacity) for theentire system, or of the utilization efficiency of the RF spectrum;another is the minimization of the worst-case delay. As multiple typesof traffic with different performance requirements are combined intopacket streams that compete for the same transmission medium, amulti-objective optimization is required.

[0038] Ideally, one would want a multiple access protocol that iscapable of effecting packet transmission scheduling as close to theoptimal scheduling as possible, but with distributed control.Distributed control implies both some knowledge of the attributes of thecompeting packet sources and limited control mechanisms.

[0039] To apply any scheduling algorithm in random multiple access, amechanism must exist that imposes an order in which packets will seizethe medium. For distributed control, this ordering must be achievedindependently, without any prompting or coordination from a controlnode. Only if there is a reasonable likelihood that packet transmissionswill be ordered according to the scheduling algorithm can one expectthat the algorithm's proclaimed objective will be attained.

[0040] The above-cited, copending U.S. patent application by MathildeBenveniste, entitled “Tiered Contention Multiple Access (TCMA): A Methodfor Priority-Based Shared Channel Access,” describes the TieredContention Multiple Access (TCMA) distributed medium access protocolthat schedules transmission of different types of traffic based on theirQoS service quality specifications. This protocol makes changes to thecontention window following the transmission of a frame and therefore isalso called Extended-DCF (E-DCF). During the contention window, thevarious stations on the network contend for access to the network. Toavoid collisions, the MAC protocol requires that each station first waitfor a randomly chosen time period, called an arbitration time. Sincethis period is chosen at random by each station, there is lesslikelihood of collisions between stations. TCMA uses the contentionwindow to give higher priority to some stations than to others.Assigning a short contention window to those stations that should havehigher priority ensures that, in most cases, the higher-prioritystations will be able to transmit ahead of the lower-priority stations.TCMA schedules transmission of different types of traffic based on theirQoS service quality specifications. A station cannot engage in backoffcountdown until the completion of an idle period of length equal to itsarbitration time.

[0041] The above-cited, copending U.S. patent application by MathildeBenveniste also applies TCMA to the use of the wireless access point asa traffic director. This application of the TCMA protocol is called thehybrid coordination function (HCF). In HCF, the access point uses apolling technique as the traffic control mechanism. The access pointsends polling packets to a succession of stations on the network. Theindividual stations can reply to the poll with a packet that containsnot only the response, but also any data that needs to be transmitted.Each station must wait to be polled. The access point establishes apolling priority based on the QoS priority of each station.

[0042] What is needed in the prior art is a way to reduce interferencebetween overlapping first and second wireless LAN cells contending forthe same medium.

SUMMARY OF THE INVENTION

[0043] In accordance with the invention, a cyclic prioritized multipleaccess (CPMA) method is disclosed which includes Fixed DeterministicPost-Backoff. Fixed deterministic post-backoff reduces conflicts betweenaccess points of overlapping cells. Contention-free sessions (CFSs) canbe generated, one from each overlapping cell. Each active access pointengages in a fixed deterministic post-backoff. A fixed deterministicbackoff delay (Bkoff times a fixed number of idle time slots) is used byall access points, with the value of Bkoff being greater than the numberof overlapping cells. The Bkoff should be large enough to enable thetraffic that needs to be accommodated by the channel. Each access pointhas a backoff timer that is counted down using the shortest interframespace possible, typically the Priority Interframe Space (PIFS). Acontention-free session (CFS) is initiated when the backoff timerexpires, and it is then reset to the value of Bkoff to start a newcycle. A cycle is measured in terms of idle time slots instead of afixed time interval. Contention-based transmissions can be attempted byan access point or other stations in the cell using their assignedpriority while the access point is counting down its backoff timer. Anew access point can get started and resolve possible collisions by asmall random backoff. Subsequent contention-free sessions (CFSs) willnot conflict, given an existing sequence of non-conflicting CFSs, sincethe follower access point's backoff delay exceeds that of the leader'sby at least one times the fixed number of idle time slots. In thismanner, contention-free sessions can be conducted without interferencein the first and second cells.

[0044] The cyclic prioritized multiple access (CPMA) method alsoincludes a staggered startup method to reduce interference betweenoverlapping first and second wireless LAN cells contending for the samemedium. Each cell includes a respective plurality of member stations. Afirst member station in the first cell coordinates a periodic sequenceof first contention-free sessions (CFS). Each contention-free sessionincludes multiple bursts with other member stations in the first cell.The first member station retains control of the medium by usinginterframe spaces sufficiently short between the bursts so that themultiple bursts appear to contending stations to be a single instance ofactivity in the medium during a session until an end of a session. Asecond member station in the second cell listens to the activity in themedium and detects an end to one of the first contention-free sessionsindicated by an interval longer than a PIFS idle interval following anend to the activity in the medium. The second member station then sets apost-backoff delay of a minimal interval following the firstcontention-free sessions of the first member station. The second memberstation then coordinates in the second cell a periodic sequence ofsecond contention-free sessions (CFS). Each of the sessions includesmultiple bursts with other member stations in the second cell. Thesecond member station retains control of the medium by using interframespaces sufficiently short between the bursts that the multiple burstsappear to contending stations to be a single instance of activity in themedium during a session until an end of a session. In this manner,contention-free sessions are interleaved on a periodic basis in thefirst and second cells.

DESCRIPTION OF THE FIGURES

[0045]FIGS. 1 through 1I show the interaction of two wireless LAN cellswhich have overlapping access points contending for the same medium, inaccordance with the invention.

[0046]FIG. 2A shows the IEEE 802.11 packet structure for a Shieldpacket, in accordance with the invention.

[0047]FIG. 2B shows the IEEE 802.11 packet structure for a beaconpacket, including the increment to the NAV period and the CFTR period.

[0048]FIG. 3 illustrates a timing diagram for the transmission of theshield packet.

[0049]FIG. 4 shows a timing diagram of a sample contention-free session(CFS) structure, which includes the shield packet, the beacon packet,and the exchange of data packets during the contention-free period shownin FIGS. 1, 1A through 1C.

[0050]FIG. 5 shows a timing diagram of non-conflicting contention-freesessions (CFS) for access point 152 (AP1) and access point 102 (AP2).

[0051]FIG. 6 shows a timing diagram of how access point 102 (AP2)listens for a PIFS idle following a busy channel and then startstransmitting a minimal interval after the contention-free session (CFS)for access point 152 (AP1).

[0052]FIG. 7 shows a timing diagram of the successful startup of accesspoint 102 (AP2) after the contention-free session (CFS) for access point152 (AP1).

[0053]FIG. 8 shows a timing diagram of access point 102 (AP2)transmitting a peg packet when it has no data to transmit in order tomaintain contiguity of its timing position in the periodic sequence ofcontention-free sessions (CFS) in the transmission order of access point152 (AP1), access point 102 (AP2), and a third access point (AP3).

[0054]FIG. 9 shows a timing diagram illustrating the result of accesspoint 102 (AP2) retiring from the periodic sequence of contention-freesessions (CFS) shown in FIG. 8, which results in a gap of long enoughduration to inadvertently permit a DCF wireless station 104B to begincontention for the channel and transmit a packet that collides with theperiodic beacon packet of AP3.

[0055]FIG. 10 shows a timing diagram illustrating that when a periodicsequence of contention-free sessions (CFS) have intervals no longer thanPIFS separating them, only the first contention-free session (CFS) hasany probability of colliding with a DCF wireless station contending forthe channel.

DISCUSSION OF THE PREFERRED EMBODIMENT

[0056] The invention disclosed broadly relates to telecommunicationsmethods and more particularly relates to wireless cells that haveoverlapping stations contending for the same medium. An inter-cellcontention-free period value is assigned to a first access point stationin the first cell, associated with an accessing order in the medium formember stations in the first and second cells. The access point in thefirst cell transmits an initial shield packet to deter other stationsfrom contending for the medium. The access point then transmits a beaconpacket containing the inter-cell contention-free period value to memberstations in the second cell. A second access point in the second cellcan then delay transmissions by member stations in the second cell untilafter the inter-cell contention-free period expires. The beacon packetsent by the first access point station also includes an intra-cellcontention-free period value, which causes the member stations in thefirst cell to delay accessing the medium until polled by the firstaccess point. After the expiration of the intra-cell contention-freeperiod, member stations in the first cell may contend for the mediumbased on the quality of service (QoS) data they are to transmit, usingthe Tiered Contention Multiple Access (TCMA) protocol.

[0057] Tiered Contention Multiple Access (TCMA) protocol is applied towireless cells that have overlapping access points contending for thesame medium. Quality of service (QoS) support is provided to overlappingaccess points to schedule transmission of different types of trafficbased on the service quality specifications of the access points. Adescription of Tiered Contention Multiple Access (TCMA) protocol appliedto overlapping wireless cells is provided in the following two copendingU.S. Patent Applications, which are incorporated herein by reference:Ser. No. 09/985,257, filed Nov. 2, 2001, by Mathilde Benveniste,entitled “Tiered Contention Multiple Access (TCMA): A Method ForPriority-Based Shared Channel Access,” and Ser. No. 10/187,132, filedJun. 28, 2002, by Mathilde Benveniste, entitled “Hybrid CoordinationFunction (HCF) Access Through Tiered Contention And Overlapped WirelessCell Mitigation.”

[0058] The method assigns a first scheduling tag to a first access pointstation in a first wireless LAN cell. The scheduling tag has a valuethat determines an accessing order for the cell in a transmission frame,with respect to the accessing order of other wireless cells. Thescheduling tag value is deterministically set. The scheduling tag valuecan be permanently assigned to the access point by its manufacturer; itcan be assigned by the network administrator at network startup; it canbe assigned by a global processor that coordinates a plurality ofwireless cells over a backbone network; it can be drawn from a pool ofpossible tag values during an initial handshake negotiation with otherwireless stations; or it can be cyclically permuted in real-time, on aframe-by-frame basis, from a pool of possible values, coordinating thatcyclic permutation with that of other access points in other wirelesscells.

[0059] An access point station 152 in wireless cell 150 is connected tobackbone network 160 in FIG. 1. The access point 152 signals thebeginning of an intra-cell contention-free session (CFS) of FIGS. 3 and4 for member stations 154A and 154B in its cell by transmitting a shieldpacket 118 during the period from T0 to T1. The shield packet 118 or 119is a short packet, such as a Physical Layer Convergence Procedure (PLCP)header without the MAC data, as shown in FIG. 2A. The shield packet 118makes the wireless channel appear busy to any station receiving theshield packet. This includes not only the member stations 154A and 154Bin cell 150, but also any stations in another overlapped cell, such ascell 100. Access point 102 and the stations 104A, 104B, and 106 of theoverlapped cell 100 also receive the shield packet 118. All suchstations listen to the channel; and when they receive the shield packet118, they defer transmitting on what they perceive to be a busy channel.The transmitting access point 152 is thus assured that no other stationwill begin contending for the medium while the access point 152 issending a beacon packet in the next step, shown in FIG. 1A. A timingdiagram for the transmission of the shield packet to begin theintra-cell contention-free session (CFS) is shown in FIGS. 3 and 4.

[0060]FIG. 2A shows the IEEE 802.11 packet structure 360 for a shieldpacket 118. The shield packet structure 360 includes fields 361 to 367.Field 365 is the PLCP header and field 367 is the empty frame body.

[0061]FIG. 1A shows the access point 152 of cell 150 transmitting thebeacon packet 124 during the period from T1 to T2. The beacon packet124, shown in FIG. 2B, includes two contention-free period values. Thefirst is the Network Allocation Vector (NAV) (or alternately itsincremental value ΔNAV), which specifies a period value P3 for theintra-cell contention-free period (CFP) for member stations in its owncell 150. The intra-cell contention-free period (CFP) is the duration ofthe contention-free session (CFS) shown in FIG. 4. Member stationswithin the cell 150 must wait for the period P3 before beginning theTiered Contention Multiple Access (TCMA) procedure. The othercontention-free period value included in the beacon packet 124 is theInter-BSS Network Allocation Vector (IBNAV), which specifies thecontention-free time response (CFTR) period P4. The contention-free timeresponse (CFTR) period P4 gives notice to any other cell receiving thebeacon packet, such as cell 100, that the first cell 150 has seized themedium for the period of time represented by the value P4. A timingdiagram for the transmission of the beacon packet is shown in FIG. 4.

[0062] The beacon packet 124 is received by the member stations 154A(with a low QoS requirement 164A) and 154B (with a high QoS requirement164B) in the cell 150 during the period from T1 to T2. The memberstations 154A and 154B store the value of ΔNAV=P3 and begin countingdown that value during the contention-free period of the cell 150. Theduration of the intra-cell contention-free period ΔNAV=P3 isdeterministically set. The member stations in the cell store theintra-cell contention-free period value P3 as the Network AllocationVector (NAV). Each member station in the cell 150 decrements the valueof the NAV in a manner similar to other backoff time values, duringwhich it will delay accessing the medium. FIG. 2B shows the IEEE 802.11packet structure 260 for the beacon packet 124 or 120, including theincrement to the NAV period and the CFTR period. The value P4 specifiesthe Inter-BSS Network Allocation Vector (IBNAV), i.e., thecontention-free time response (CFTR) period that the second access point102 must wait, while the first cell 150 has seized the medium. Thebeacon packet structure 260 includes fields 261 to 267. Field 267specifies the ΔNAV value of P3 and the CFTR value of P4. The methodassigns to the first access point station a first inter-cellcontention-free period value, which gives notice to any other cellreceiving the beacon packet that the first cell has seized the mediumfor the period of time represented by the value. The inter-cellcontention-free period value is deterministically set.

[0063] If the cells 100 and 150 are mostly overlapped, as in region 170shown in FIG. 1A, then transmissions from any one station in one cell150 will be received by most or all stations in the overlapped cell 100.The beacon packet 124 transmitted by the access point 152 in cell 150 isreceived by all of the stations in cell 150 and all of the stations incell 100, in FIG. 1A.

[0064] Alternately, if only one or a small portion of stations are inthe region of overlap 170, then a contention-free time response (CFTR)packet will be used to relay the information in the beacon packet tothose stations remote from the transmitting station. The description ofthe CFTR packet and its operation is provided in the copending U.S.patent application Ser. No. 10/187,132, filed Jun. 28, 2002, by MathildeBenveniste, entitled “Hybrid Coordination Function (HCF) Access ThroughTiered Contention And Overlapped Wireless Cell Mitigation,” incorporatedherein by reference. For a partially overlapped region 170, any stationreceiving the beacon packet 124 immediately rebroadcasts acontention-free time response (CFTR) packet containing a copy of thefirst inter-cell contention-free period value P4. The value P4 specifiesthe Inter-BSS Network Allocation Vector (IBNAV), i.e., thecontention-free time response (CFTR) period that the second access point102 must wait while the first cell 150 has seized the medium. In thismanner, the notice is distributed to the second access point station 102in the overlapping second cell 100.

[0065]FIG. 1B shows the point coordinator in access point 152 of cell150 controlling the contention-free period within the cell 150 by usingthe polling packet “D1” 128 during the period from T2 to T3. A timingdiagram for the transmission of the polling packet is shown in FIG. 4.In the mean time, the second access point 102 in the second cell 100connected to backbone network 110 stores the first inter-cellcontention-free period value P4 received in the CFTR packet 126, whichit stores as the Inter-BSS Network Allocation Vector (IBNAV). The secondaccess point 102 decrements the value of IBNAV in a manner similar toother backoff time values, during which it will delay accessing themedium. FIG. 1C shows the wireless station 154A in cell 150 respondingto the polling packet 128 by returning a responsive data packet “U1”140. A timing diagram for the transmission of the responsive data packet“U1” is shown in FIG. 4. Subsequent, similar exchanges in cell 150 areshown in FIG. 4, where access point 152 sends the polling packet “D2”and the polled station in cell 150 responds with data packet “U2”.Access point 152 then sends the polling packet “D3”, but there is noresponse from the polled station in cell 150; so within a PIFS interval,access point 152 sends the polling packet “D4” and the polled station incell 150 responds with data packet “U4”. It is seen at this point inFIG. 1D and FIG. 4 that the NAV value has been counted down to zero inthe stations of cell 150, signifying the end of the contention-freesession (CFS) for cell 150. FIG. 1D also shows that the IBNAV value inthe access point 102 and the CFTR value in the other stations of theoverlapped cell 100 have also been counted down to zero. The secondaccess point 102 in the cell 100 takes this as its cue to transmit ashield packet 119 to begin a contention-free session (CFS) for cell 100.

[0066] The method similarly assigns to the second access point 102station in the second wireless LAN cell 100 that overlaps the first cell150 a second contention-free period value CFTR=P7 longer than the firstcontention-free period value CFTR=P4. FIG. 1D shows the second accesspoint 102 in the cell 100 transmitting a shield packet 119 during theperiod from T4 to T5. The shield packet 119 is a short packet, such as aPhysical Layer Convergence Procedure (PLCP) header without the MAC data,as shown in FIG. 2A. The shield packet 119 makes the wireless channelappear busy to any station receiving the shield packet. This includesnot only the member stations 104A, 104B, and 106 in cell 100, but alsoany stations in another overlapped cell, such as cell 150. The accesspoint 152 and stations 154A and 154B of the overlapped cell 150 alsoreceive the shield packet 119. All such stations receiving the shieldpacket 119 delay transmitting on what they perceive to be a busychannel. The transmitting access point 102 is thus assured that no otherstation will begin contending for the medium while the access point 102is sending a beacon packet in the next step, shown in FIG. 1E.

[0067] Access point 102 in cell 100 sends its beacon packet 120 in FIG.1E, including its contention-free period values of NAV (P6) and IBNAV(P7), to the member stations 104A (with a low QoS requirement 114A),104B (with a high QoS requirement 114B) and 106 in the cell 100 duringthe period from T5 to T6. The stations 152, 154A, and 154B of theoverlapped cell 150 also receive the beacon packet 120. FIG. 1F showsthe point coordinator in access point 102 of cell 100 controlling thecontention-free period within cell 100 using the polling packet 132during the period from T6 to T7. FIG. 1G shows the wireless station 104Bin cell 100 responding to the polling packet 132 by returning aresponsive data packet 142. It is seen at this point in FIG. 1H that theNAV value has been counted down to zero in the stations of cell 100,signifying the end of the contention-free session (CFS) for cell 100.FIG. 1H also shows that the IBNAV value in the access point 152 and theCFTR value in the other stations of the overlapped cell 150 have alsobeen counted down to zero. All of the stations in both cells 100 and 150have their NAV and CFTR/IBNAV values at zero, and they take this astheir cue to begin the contention period.

[0068] The method uses the Tiered Contention Multiple Access (TCMA)protocol to assign to first member stations in the first cell 150 afirst shorter backoff value for high quality of service (QoS) data and afirst longer backoff value for lower QoS data. FIG. 1H shows the station154B in the cell 150, having a high QoS requirement 164B, decreasing itshigh QoS backoff period to zero and beginning TCMA contention. Station154B transmits a request-to-send (RTS) packet 144 to station 154A duringthe period from T8 to T9. Station 154A responds by sending aclear-to-send (CTS) packet to station 154B.

[0069] Then, station 154B transmits its high QoS data packet 130 duringthe period from T9 to T10 in FIG. 1I. The backoff time is the intervalthat a member station waits after the expiration of the contention-freeperiod P3 before the member station 154B contends for access to themedium. Since more than one member station in a cell may be competingfor access, the actual backoff time for a particular station can beselected as one of several possible values. In one embodiment, theactual backoff time for each particular station is deterministicallyset, so as to reduce the length of idle periods. In another embodiment,the actual backoff time for each particular station is randomly drawnfrom a range of possible values between a minimum delay interval to amaximum delay interval. The range of possible backoff time values is acontention window. The backoff values assigned to a cell may be in theform of a specified contention window. High QoS data is typicallyisochronous data, such as streaming video or audio data, that mustarrive at its destination at regular intervals. Low QoS data istypically file transfer data and email, which can be delayed in itsdelivery and yet still be acceptable. The Tiered Contention MultipleAccess (TCMA) protocol coordinates the transmission of packets within acell so as to give preference to high QoS data over low QoS data toinsure that the required quality of service is maintained for each typeof data.

[0070] The method uses the Tiered Contention Multiple Access (TCMA)protocol to assign to second member stations in the second cell 100 asecond shorter backoff value for high QoS data and a second longerbackoff value for lower QoS data.

[0071] The first and second cells are considered to be overlapped whenone or more stations in the first cell can inadvertently receive packetsfrom member stations or the access point of the other cell. Theinvention reduces the interference between the overlapped cells bycoordinating the timing of their respective transmissions whilemaintaining the TCMA protocol's preference for the transmission of highQoS data over low QoS data in each respective cell.

[0072]FIG. 3 shows a timing diagram for the transmission of the shieldpacket. A CFS is started with the shield packet, which is a short frame(e.g., Physical Layer Convergence Procedure (PLCP) header without MACdata). The AP will wait for an idle period of PIFS to transmit followingthe shield. If an (E)DCF transmission collides with the shield, the APwill hear the transmission and defer initiation of the CFS body. Aftercompletion of the (E)DCF transmission, the CFS will start, following aPIFS idle. Transmission of the shield before the CFS body is not alwaysneeded. For example, it is not needed if the AP knows that the idle gapbetween the CFS and the previous transmission is equal to PIFS—i.e.,when the backoff delay is 1—during the last busy period.

[0073]FIG. 4 shows a timing diagram of a sample CFS structure. Itincludes the shield packet, the beacon packet, and the exchange of datapackets during the contention-free period shown in FIGS. 1, 1A through1C.

[0074] All stations listen to the channel; and when they receive theshield packet, they defer transmitting on what they perceive to be abusy channel. The transmitting access point is thus assured that noother station will begin contending for the medium while the accesspoint is sending a beacon packet. If another station and the accesspoint have simultaneously begun transmission, then the benefit of theshield packet is that the other station's (E)DCF transmissions collidingwith the shield packet will cause postponement of the start of the CFSbody by the access point until the channel is clear. The CFS is thusassured of no (E)DCF conflict because of its shorter ArbitrationInterframe Space (AIFS). While the other station's colliding (E)DCFtransmission is unsuccessful, the CFS body will be transmitted later bythe access point without conflict. Channel time is saved this way ifCFSs are longer than DCF transmissions. This method can also be appliedto PCFSs if there is no other mechanism to protect them from collisionswith (E)DCF transmissions, as there is in the point coordinationfunction (PCF). Still further, a special shield packet may also be usedin Inter-BSS NAV protection.

[0075] The following definitions are believed to be helpful to anunderstanding of the invention.

[0076] Contention-free burst (CFB): A technique for reducing MAC layerwireless medium (WM) access overhead and susceptibility to collisions,in which a single station may transfer a plurality of MAC protocol dataunits (MPDUs) during a single transmission opportunity (TXOP), retainingcontrol of the WM by using interframe spaces sufficiently short that theentire burst appears to be a single instance of WM activity tocontending stations.

[0077] Contention-free session (CFS): Any frame exchange sequence thatmay occur without contention following a successful channel accessattempt. A CFS may involve one or more stations. A CFS may be initiatedby any station. A contention-free burst (CFB) and an RTS/CTS exchangeare both examples of a CFS. A contention-free burst (CFB) is a specialcase of a contention-free session (CFS) that is started by a hybridcoordinator (HC).

[0078] Contention-free period (CFP): A time period during operation of abasic service set (BSS) when a point coordination function (PCF) orhybrid coordination function (HCF) is used, and transmissionopportunities (TXOPs) are assigned to stations by a point coordinator(PC) or hybrid coordinator (HC), allowing frame exchanges to occurwithout inter-station contention for the wireless medium (WM) and atregular time intervals. The contention-free period (CFP) is the durationof a contention-free session (CFS).

[0079] Periodic contention-free session (PCFS): A contention-freesession (CFS) that must occur at regular time intervals. Acontention-free period (CFP) is an example of a PCFS. Both PCFSs andCFSs are needed, the PCFSs used for periodic traffic and the CFSsproviding efficient use of channel time, as channel availabilitypermits. When restricting the time to the next access attempt, thechannel cannot be used sooner, even if needed and available; it limitsefficiency of channel re-use.

[0080] The description of the invention can be simplified by consideringthat CFSs/PCFSs are initiated by access points (AP). However, CFSs/PCFSscan be initiated by any station, whether or not it is an AP.

[0081] In a multi-BSS system, there will still be interference betweenBSSs assigned the same channel, once channels have been assigned to theBSSs. Allocation of the channel time is achieved through dynamicbandwidth allocation, which enables sharing of the channel time amongco-channel BSSs efficiently so that no channel time is left idle.Because there is no central controller, distributed, prioritized,dynamic bandwidth allocation algorithms are needed in order tocoordinate multi-BSS channel reuse.

[0082] A contention-free burst (CFB) is a special case of acontention-free session (CFS) that is started by a hybrid coordinator(HC). It would be desirable that CFSs/PCFSs have priority access over(E)DCF transmissions. It would also be desirable for (E)DCFtransmissions to access the channel at an assigned priority. It wouldstill further be desirable for CFSs to be able to regain control of thechannel periodically and conflict-free. It would also be desirable thatthere are no conflicts with CFSs from other BSSs or (E)DCFtransmissions. Finally, it would be desirable to have efficient channelre-use (with no channel left idle) that is greater than or equal to thedynamic bandwidth allocation.

[0083] Other workers in the field have proposed that (E)DCF can be usedby the CFSs to access the channel. The CFSs would be placed in thehighest priority class which is above the highest priority in (E)DCF.Shorter AIFS would be used for CFS access, which helps avoid collisionswith (E)DCF transmissions. Backoff would help deal with CFS conflictsamong BSSs. However, such proposals raise concerns about random backoff.With a random backoff, (E)STAs may access the channel before the HC,since (PIFS+1)=DIFS allows a (E)DCF station to transmit. Also, a longbackoff leaves many idle slots, which would allow (E)STAs to transmitbefore HCs. Furthermore, a short backoff causes collisions between CFSs.Such proposals also raise concerns about fixed or minimum re-visit time.When restricting the time to the next access attempt, the channel cannotbe used sooner, even if needed and available. Also, although this may bea good approach for PCFSs, it limits efficiency of channel re-use.

[0084] These problems are avoided by the cyclic prioritized multipleaccess (CPMA) method for contention-free sessions (CFS). The cyclicprioritized multiple access (CPMA) method includes three features:

[0085] 1—Fixed Deterministic Post-Backoff, which reduces conflictsbetween APs.

[0086] 2—Staggered Start-up, which provides contiguous sequences of CFSsto deter collisions with (E)STAs. (Staggered Start-up is an optionalfeature.)

[0087] 3—‘Pegging’, which preserves CFS sequence contiguity. (‘Pegging’is an optional feature.)

[0088] The cyclic prioritized multiple access (CPMA) method requires amechanism for ‘busy’ channel detection (detection of the start and endof a CFS), such as the Inter-BBS NAV. This is described in the copendingU.S. patent application Ser. No. 10/187,132, filed Jun. 28, 2002, byMathilde Benveniste, entitled “Hybrid Coordination Function (HCF) AccessThrough Tiered Contention And Overlapped Wireless Cell Mitigation,”which is incorporated by reference. The cyclic prioritized multipleaccess (CPMA) method also requires fully overlapped BSSs or partiallyoverlapping BSSs with IBNAV protection. This is also described in thecopending U.S. patent application Ser. No. 10/187,132. An alternative tosuch protection is ‘parallel’ backoff to avoid “Neighborhood Capture”,as is described in the copending regular U.S. patent application Ser.No. ______, filed ______, 2002, by Mathilde Benveniste, entitled“Wireless LANs And Neighborhood Capture,” which is incorporated byreference.

[0089] The Fixed Deterministic Post-Backoff feature of cyclicprioritized multiple access (CPMA) reduces conflicts between accesspoints of overlapping cells. Contention-free sessions (CFSs) can begenerated, one from each overlapping cell. Each active access pointengages in a fixed deterministic post-backoff. A fixed deterministicbackoff delay (Bkoff times a fixed number of idle time slots) is used byall access points, with the value of Bkoff being greater than the numberof overlapping cells. The Bkoff should be large enough to enable thetraffic that needs to be accommodated by the channel. Each access pointhas a backoff timer that is counted down using the shortest interframespace possible (typically PIFS). A contention-free session (CFS) isinitiated when the backoff timer expires, and it is then reset to thevalue of Bkoff to start a new cycle. A cycle is measured in terms ofidle time slots instead of a fixed time interval. Contention-basedtransmissions can be attempted by an access point or other stations inthe cell using their assigned priority while the access point iscounting down its backoff timer. A new access point can get started andresolve possible collisions by a small random backoff. Subsequentcontention-free sessions (CFSs) will not conflict, given an existingsequence of non-conflicting CFSs, since the follower access point'sbackoff delay exceeds that of the leader's by at least one times thefixed number of idle time slots. In this manner, contention-freesessions can be conducted without interference in the first and secondcells.

[0090]FIG. 5 shows a timing diagram of non-conflicting contention-freesessions (CFS) for access point 152 (AP1) and access point 102 (AP2).The figure shows the CFSs repeating in cycles. Contention-free sessions(CFSs) are generated, one from each overlapping BSS. Each active APengages in fixed deterministic post-backoff, which is characterized bythe post-backoff being ON. A fixed deterministic backoff delay, Bkoff,is used by all APs, with Bkoff greater than the number of overlappingBSSs. The Bkoff should be large enough to enable the traffic that can beaccommodated by the channel. FIG. 5 shows that the channel is accessedand the backoff timer is counted down using the shortest AIFS possible.A CFS is initiated when backoff expires and the backoff is reset toBkoff, which starts a new cycle. A cycle is measured in terms of idletime slots; it does not represent a fixed time interval. (E)DCFtransmissions are attempted by their assigned priority while the AP iscounting its backoff down. A new AP can get started and resolve possiblecollisions by a small random backoff. FIG. 5 shows that subsequent CFSswill not conflict, given a sequence of non-conflicting CFSs. Becausetheir previous CFSs did not conflict, the follower AP's backoff delayexceeds that of the leader's by at least one.

[0091] In general, the cyclic prioritized multiple access (CPMA) methodcan get started by a random backoff, 0, 1 . . . [small value]. To reducethe probability of collisions with (E)DCF transmissions, a contiguoussequence of CFBs (consecutive CFBs separated by idle gaps <=PIFS) isgenerated by observing the following startup procedure. If there is noother AP present, then the first AP will get started after waiting for acycle, which is the time it takes Bkoff idle time slots to expire (withdeferral time).

[0092]FIG. 6 shows a timing diagram of how access point 102 (AP2)listens for a PIFS idle following a busy channel and then startstransmitting a minimal interval after the contention-free session (CFS)for access point 152 (AP1). If one AP is operating and a second APpowers on, AP2 listens to the channel until it observes a PIFS Idlefollowing a busy channel, or for another indication of a CFS. Then itlooks for the first idle longer than PIFS and sets its post-backoffdelay to transmit always right after AP1. An idle period X=PIFS+x, x>b0, has been detected at time t; AP2's backoff at time t is set atBkoff−x. If several APs power on during the same cycle, collisionbetween new APs is possible. It can be resolved by a random backoff, 0,11 . . . [small value]. As is shown in FIG. 6, AP2 listens to thechannel until it observes a PIFS Idle following a busy channel. Then itlooks for the first idle longer than PIFS and sets its post-backoffdelay to transmit always right after AP1. When an idle periodDIFS=PIFS+1 has been detected at time t, AP2's backoff at time t is setat Bkoff−1.

[0093]FIG. 7 shows a timing diagram of the successful startup of accesspoint 102 (AP2) after the contention-free session (CFS) for access point152 (AP1). These are non-conflicting, contiguous CFSs. Subsequent CFSswill be contiguous, given a sequence of contiguous CFSs. Because theirprevious CFSs were contiguous, the follower AP's backoff delay exceedsthat of the leader's by exactly one. NAV protection and longer AIFSprevent DCF transmissions from conflicting with new CFSs.

[0094]FIG. 8 shows a timing diagram of access point 102 (AP2)transmitting a peg packet when it has no data to transmit in order tomaintain contiguity of its timing position in the periodic sequence ofcontention-free sessions (CFS) in the transmission order of access point152 (AP1), access point 102 (AP2), and a third access point (AP3).Pegging maintains contiguity. If an AP has no traffic, it will transmita short pegging packet and set its backoff=Bkoff . In this manner, nogaps of length DIFS+1 are left idle, and thus (E)DCF stations cannotseize the channel until all APs have completed one CFS per cycle.

[0095]FIG. 9 shows a timing diagram illustrating the result of accesspoint 102 (AP2) retiring from the periodic sequence of contention-freesessions (CFS) shown in FIG. 8, which results in a gap of long enoughduration to inadvertently permit a DCF wireless station 104B to begincontention for the channel and transmit a packet that collides with theperiodic beacon packet of AP3. If an AP retires or does not use pegging,gaps or idle periods >PIFS will occur. Then, a collision between itsfollower AP and an (E)DCF station is possible. Newly activated APs canhelp take up excess void (two retirements back to back) in the sequence.CFSs can be protected from (E)DCF transmissions by using a shieldpacket, as described in the copending U.S. patent application Ser. No.______, filed ______, 2002, by Mathilde Benveniste, entitled “‘Shield’:Protecting High Priority Channel Access Attempts In Overlapped WirelessCells,” which is incorporated by reference.

[0096]FIG. 10 shows a timing diagram illustrating that, when a periodicsequence of contention-free sessions (CFS) have intervals no longer thanPIFS separating them, only the first contention free session (CFS) hasany probability of colliding with a DCF wireless station contending forthe channel. Given a contiguous sequence of CFSs (no gaps due toretirements), only the first AP can collide with (E)DCF stations. Inlighter traffic, this is a low probability. Subsequent APs do notcollide with (E)DCF stations because of their shorter AIFS. CFSs can beprotected from (E)DCF transmissions by using a shield packet. If an APexperiences a collision with (E)DCF transmission, it may reset itsbackoff=Bkoff−x, thus moving from the head of the sequence behindanother AP. Another AP will thus take the lead position, sharing thecollision probability.

[0097] In summary, the cyclic prioritized multiple access (CPMA) methodincludes the fixed deterministic post-backoff feature, which preventscollisions among the different APs. The staggered start-up featureachieves contiguous CFS sequences.

[0098] Contiguity decreases the probability of collision with (E)DCFtransmissions, since no idle gaps are left of the size of the AIFS of(E)DCF stations. The ‘pegging’ feature maintains contiguity of CFSsequences. If collisions occur, a small random backoff is used toresolve conflicts. The probability of conflicts is still less than withsimple random backoff.

[0099] PCFSs provide regular access to the channel for periodic traffic.The use of PCFSs alone cannot not provide efficient dynamic bandwidthallocation. CFSs generated on a contention basis must complement PCFSs.PCFSs and CFSs access the channel with the shortest AIFS. Quality ofservice (QoS) can be managed while using the cyclic prioritized multipleaccess (CPMA) method by an AP scheduling traffic as follows: periodictraffic is transmitted in PCFSs; non-periodic traffic is placed eitherin a PCFS or in its allotted CFSs according to traffic priority;delay-sensitive traffic is scheduled first, followed by traffic of lowerpriorities.

[0100] Various illustrative examples of the invention have beendescribed in detail. In addition, however, many modifications andchanges can be made to these examples without departing from the natureand spirit of the invention.

What is claimed is:
 1. A method for reducing interference betweenoverlapping first and second wireless LAN cells in a medium, each cellincluding a respective plurality of member stations, comprising:coordinating by a first member station in the first cell a firstcontention-free session, said session including multiple bursts withother member stations in the first cell, and retaining control of themedium by said first member station by using interframe spacessufficiently short between the bursts that the multiple bursts appear tocontending stations to be a single instance of activity in the mediumduring a session until an end of a session; setting by the first memberstation a backoff timer to a fixed deterministic post-backoff delay,which has a value of Bkoff times a fixed number of idle time slots, thevalue of Bkoff being greater than a number of overlapping cells;counting down the backoff timer by the first member station;transmitting a second contention-free session by the first memberstation when the backoff timer expires; and resetting the backoff timerto the value of Bkoff to start a new cycle.
 2. The method of claim 1,which further comprises: said backoff timer being counted down using ashortest possible interframe space.
 3. The method of claim 2, whichfurther comprises: said interframe space being a Priority InterframeSpace (PIFS).
 4. The method of claim 2, which further comprises: saidinterframe space being a minimum arbitration interframe space (AIFS). 5.The method of claim 1, which further comprises: listening by a secondmember station in the second cell to said activity in the medium anddetecting an end to one of said first contention-free sessions indicatedby an interval longer than a PIFS idle interval following an end to saidactivity in the medium; setting a second post-backoff delay by saidsecond member station to transmit a minimal interval after said firstcontention-free session of said first member station; and coordinatingby said second member station in the second cell a secondcontention-free session, said second session including multiple burstswith other member stations in the second cell, and retaining control ofthe medium by said second member station by using interframe spacessufficiently short between the bursts that the multiple bursts appear tocontending stations to be a single instance of activity in the mediumduring a session until an end of a session.
 6. The method of claim 5,which further comprises: said second post-backoff delay being a minimumarbitration interframe space (AIFS).
 7. The method of claim 5, whichfurther comprises: initiating by said second member station acontention-free session when said second post-backoff delay is counteddown to zero; and resetting said second post-backoff delay to start anew cycle.
 8. The method of claim 5, which further comprises: separatingconsecutive ones of said first and second contention-free sessions byidle gaps which are less than or equal to a Priority Interframe Space(PIFS).
 9. The method of claim 8, in which said setting said secondpost-backoff delay further comprises: detecting an idle period X=PIFS+x,x>0 at time t; and setting said second post-backoff delay at time t, toa value of Bkoff−x.
 10. A wireless communications system having reducedinterference between overlapping first and second wireless LAN cells ina medium, each cell including a respective plurality of member stations,comprising: a first access point station in the first cell; said firstaccess point station coordinating in the first cell a firstcontention-free session, said session including multiple bursts withother member stations in the first cell, and retaining control of themedium by said first access point by using interframe spacessufficiently short between the bursts that the multiple bursts appear tocontending stations to be a single instance of activity in the mediumduring a session until an end of a session; a backoff timer in saidfirst access point station, which is set to a fixed deterministicpost-backoff delay, which has a value of Bkoff times a fixed number ofidle time slots, the value of Bkoff being greater than a number ofoverlapping cells; said backoff timer counting down said fixeddeterministic post-backoff delay; said first access point stationtransmitting a second contention-free session when the backoff timerexpires; and said backoff timer resetting the value of Bkoff to start anew cycle.
 11. The system of claim 10, which further comprises: saidbackoff timer being counted down using a shortest possible interframespace.
 12. The system of claim 1, which further comprises: saidinterframe space being a Priority Interframe Space (PIFS).
 13. Thesystem of claim 11, which further comprises: said interframe space beinga minimum arbitration interframe space (AIFS).
 14. The system of claim10, which further comprises: a second access point station in the secondcell; said second access point listening in the second cell to saidactivity in the medium and detecting an end to said firstcontention-free session indicated by an interval longer than a PIFS idleinterval following an end to said activity in the medium; said secondaccess point setting a second post-backoff delay to a minimal intervalto transmit after said first contention-free session of said firstaccess point; and said second access point coordinating in the secondcell a second-contention free session, said second session includingmultiple bursts with other member stations in the second cell, andretaining control of the medium by said second access point by usinginterframe spaces sufficiently short between the bursts that themultiple bursts appear to contending stations to be a single instance ofactivity in the medium during a session until an end of a session. 15.The system of claim 14, which further comprises: said secondpost-backoff delay being a minimum arbitration interframe space (AIFS).16. The system of claim 14, which further comprises: said second accesspoint initiating a contention-free session when said second post-backoffdelay is counted down to zero; and said second access point resettingsaid second post-backoff delay to start a new cycle.
 17. The system ofclaim 14, which further comprises: said second access point separatingconsecutive ones of said first and second contention-free sessions byidle gaps which are less than or equal to a Priority Interframe Space(PIFS).
 18. The system of claim 14, in which said setting a secondpost-backoff delay further comprises: said second access point detectingan idle period X=PIFS+x, x>0 at time t; and said second access pointsetting said post-backoff delay at time t, to a value of Bkoff−x.